The typical electrical power supply system for automotive vehicles is a so-called constant voltage system having a nominal rated voltage of twelve volts. It comprises a single storage battery, a generator and a regulator to maintain the generator output voltage and current within certain limits. The generator, which typically takes the form of an alternator with a rectifier, is driven at variable speed by the vehicle engine and supplies DC power to the load circuits and charges the battery. The regulator is typically a switching system for the field coil of the alternator across the battery to regulate the generator output voltage to the desired value and to limit the generator current to a desired value. Dual battery systems are also known for use in automotive vehicles. Such systems are useful in vehicles which have different load devices adapted to be energized at two different voltages. For example, the system may comprise two batteries in series with a first load circuit including load devices designed to be powered at the rated voltage of one battery and a second load circuit including load devices designed to be powered at twice the rated voltage of one battery.
The electrical load on the generator of an automotive vehicle has increased with on-going changes in vehicle design and accessories. As a result, the typical low voltage system utilizing a twelve volt battery has become increasingly inefficinet. Load devices which require high power necessarily draw heavy currents and the power losses in the generator and in the conductors are relatively high. In order to achieve a more efficient system, it is desirable to utilize load devices with a higher rated voltage. Certain load devices which require high power with large current, such as a starting motor, are preferably energized from a storage battery so it is advantageous to put two batteries in series to achieve higher output voltage. Other load devices can be operated advantageously at higher voltage and lower current and may have an adjustable power level according to the requirements on the device.
In the prior art, dual voltage systems with two batteries are known in a variety of circuit arrangements. Typically, in such systems, the load devices such as the lamps and accessories are operated at low voltage, i.e. at the rated voltage of a twelve volt battery. In some vehicle electrical systems, such as in trucks, a starter motor rated at twenty-four volts is utilized. In such systems, two twelve volt batteries are connected in series for supplying the starter motor. Various circuit arrangements are known for charging the batteries from the generator and for supplying power to the twelve volt and twenty-four volt load devices. Systems of the type utilizing two batteries to provide a dual voltage power supply are disclosed in the prior art as represented by the Seike U.S. Pat. No. 3,710,226 granted Jan. 9, 1973, Follmar U.S. Pat. No. 4,044,293 granted Aug. 23, 1977, Himmler U.S. Pat. No. 4,047,088 granted Sept. 6, 1977 and the Taylor U.S. Pat. No. 4,210,856 granted July 1, 1980. The prior art vehicle electrical systems do not provide a high voltage power supply output which is adjustable in accordance with load requirements.
My copending application Ser. No. 569,723 filed Jan. 10, 1984 entitled "DUAL VOLTAGE POWER SUPPLY SYSTEM FOR VEHICLES" provides both a regulated low voltage output and a variable high voltage output. In this dual voltage system, the low voltage output is regulated at the value required by the battery and is substantially independent of engine speed. The output of the generator which is applied to the high voltage load circuit may vary from a low voltage at idle speed of the engine to a regulated high voltage which is achieved by the generator only at higher engine speeds.
A general object of this invention is to provide a vehicle power supply with a high voltage output which is adjustable in accordance with the requirements of a load device and to overcome certain disadvantages of prior art dual voltage or plural-voltage systems.